In a single or multiple inboard power design the use of a propeller tunnel of concave curvature extending from the point of propeller shaft exit from the hull to the transom edge of the vessel at a marginally increasing gradient produces enhanced performance characteristics.
This design (1) decreases the angle of incidence between the main power source and the propeller shaft thus gaining mechanical advantage and increased physical longevity for mechanical components, (2) increases the flow of water concentrated in the area of best advantage for propeller efficiency, and (3) reduces the natural propensity of the spinning propeller to produce air pockets and bubbles known as “cavitation”. However these desirable effects are off-set by a number of disadvantages. One of these disadvantages is the effect that the propeller tunnel has on the efficiency of a semi-displacement or planing hull.
Planing hull types provide the vessel with the ability to move on or very close to the surface of the water, thus freeing the hull from the constraints of “hull speed”, a condition that slows and limits a vessels' speed proportionately to its hull length, beam and depth as it moves through the water in a plowing motion. The propeller tunnel, when configured into the hull, creates suction that inhibits the movement of the planing hull toward the surface of the water.
Another disadvantage of propeller tunnels in planing hulls relates to the position of the primary propulsion unit. While the propeller tunnel provides improved mechanical efficiency and extends the life of mechanical devices, it utilizes a long propeller shaft and the installation of the necessary support hardware, struts and cutlass bearings. The long propeller shaft and exterior hardware increase the possibility of damage due to encounters with flotsam or during grounding events.